The present invention relates generally to apparatus for machining a cylindrical shaft and, more specifically, to a shaft rounding machine which electrochemically removes material from the shaft in response to a measurement of the distance between a preselected point on the shaft's cylindrical surface and a chordal line which extends between two points on the shaft's surface.
Many applications require rotating equipment to incorporate shafts which meet very stringent noise acceptance criteria. For example, submarine power generating machinery must have exceptionally round bearing journals in order to satisfy very strict noise standards. In these types of applications, roundness is sometimes required to be better than 50 millionths of an inch, defined as the maximum allowable deviation from a perfect circle.
In common practice, roundness is achieved by first grinding the journal slightly oversize and inspecting it for roundness or deviation therefrom. Correction of this deviation is obtained by employing the process of superfinishing. The process of superfinishing was developed prior to 1940 and was originally conceived as a means of achieving a superior surface finish and not primarily as a means of rounding shafts. Superfinishing is a very slow and tedious manufacturing method and requires a considerably high level of operator expertise. Since this process involves removal of metal from the total surface, with more being removed from the deviating high points, the possibility exists for machining the journal undersized prior to achieving acceptable roundness.
There has been a great deal of development work done in the field of electrochemical deburring and machining. U.S. Pat. No. 2,590,927 was issued to Brandt et al. on Apr. 1, 1952 and describes an electrolytic method for removing the burrs from the cut edges of laminated cores. This method described in the Brandt patent utilizes phosphoric acid in conjunction with an electrical current to render the phosphoric acid active for the intended purposes of removing the above-mentioned metallic slivers and burrs. By electrically connecting the laminated core to the positive terminal of a direct current power source and disposing a negatively connected cathode proximate the machined surface, an electrical current can be passed through the electrolyte between the machined surface and the cathode. Under the influence of the electrical current, both burrs and slivers are etched away from the machined surface when subjected to the phosphoric acid solution. This use of an electrical current in combination with a phosphoric acid solution is an improvement over prior methods which utilize acid alone in the absence of an electric current such as U.S. Pat. No. 2,293,951 which was issued to Seastone et al. on Aug. 25, 1942, which describes a chemical etching operation that utilizes a solution of nitric, hydrochloric or sulfuric acid without the use of an electric current. An alternative method of treatment is described in U.S. Pat. No. 2,243,578 which was issued to Reardon on May 27, 1941 and which subjects the short-circuited laminated core to an acid phosphate treatment.
Electrochemical machining and shaping is also described in U.S. Pat. No. 3,058,895 issued to Williams on Oct. 16, 1962, and in U.S. Pat. No. 3,365,381 issued to Fromson on Jan. 23, 1968. The Williams patent describes the shaping and contouring of electrically conductive and electrochemically erodable workpieces and also discloses an apparatus for accomplishing this task. The Fromson patent discloses an apparatus for the electrolytic machining of a workpiece in which an electrode is operated in relation to the workpiece while an electrolyte fluid is passed through a fluid passage. Copending patent application Ser. No. 493,843, filed May 12, 1983 and assigned to present assignee, discloses an electrochemical deburring operation which utilizes a salt solution as its electrolyte.
Electrochemical machining or deburring generally requires the disposition of a cathode proximate the surface to be machined. This cathode is connected in electrical communication with the negative terminal of a direct current power supply and is disposed a preselected distance from the surface which is to be machined. An electrolyte is caused to flow in the gap between the cathode and the component surface and, when the subject component is connected in electrical communication with the positive terminal of a direct current power supply, a current flows between the cathode and the workpiece. This process removes material from the workpiece by causing ions to be deplated from the surface or the workpiece which is most proximate the cathode. Generally, the electrolyte is a fluid acid or salt solution which is caused to flow into the gap between the workpiece and the cathode.
The present invention provides a bracket member which is shaped to provide a two-point contact with the shaft. It should be understood that the use of the term "two-point contact" throughout the specification refers to an association of components in which the bracket and shaft, observed in a radial section view, contact each other at two points. It should be understood that each of these contact points actually describes an axial line which extends along the surface of the shaft. Therefore, the bracket of the present invention actually touches the workpiece shaft along two lines of contact. However, for purposes of describing the present invention, a more precise explanation of the present invention can be achieved by radial cross-section views of the present invention in association with a workpiece shaft and, in this form of two-dimensional presentation, the contact is at two points.
The two points of contact, described above, describe a chordal line therebetween. This chordal line extends through a portion of the shaft workpiece. Attached to the bracket of the present invention, a measuring device extends radially inward toward a preselected point on the cylindrical surface of the shaft. This measuring device can be the type which incorporate a probe that is capable of making contact with the cylindrical surface of the shaft and, by appropriately positioning the probe relative to the two points of contact, the distance between the tip of the probe (i.e. the shaft's surface) and the chordal line can be accurately determined.
The bracket of the present invention is disposed in slidable two-point contact with the shaft and the shaft is permitted to rotate while maintaining contact with the stationary bracket. As the shaft rotates, a succession of points pass under the probe of the measuring device and a number of sequential measurements can therefore be made. It should be understood that the points measured by these sequential measurements all lie on a circumferential line passing around the shaft on its surface. Although each of these points will lie at various positions around the diameter of the shaft, they all lie in a generally identical axial position on the cylindrical surface of the shaft.
The measuring device produces an electrical signal which is analogous to the distance between the point being measured at that particular time and the chordal line described above. By setting the measuring device to an appropriate bias value, the present invention can measure deviations between each of the points and a perfect circle. By setting the above-mentioned bias value to one which produces no electrical signal when a point at the shaft's minimum radius is measured, all measurements of deviation will be relative to that minimum radial dimension. Furthermore, the present invention amplifies this electrical signal and this amplified signal is transmitted to the cathode of an electrochemical machining apparatus. Since the rate of material removal is dependent on the rate of current flow between the cathode and the subject workpiece, variations in the voltage level of the cathode will result in varying rates of metal removal from the workpiece at the location of the cathode. Therefore, the present invention has the capability of selectively machining the high points, or deviations, which extend radially outward from the radius of a perfect circle around the center line of the shaft workpiece.
In order to be effective, the present invention as described above should have the cathode disposed proximate the probe of the measuring device. However, it should be apparent that the measuring device and the cathode can be disposed an arcuate distance from each other if their operation can be coordinated in such a way so as to delay the response of the amplifier a predetermined period of time which is based upon the arcuate distance between the measuring device and the cathode and also as a function of the speed of rotation of the shaft. An alternative method of coordinating the function of these two components is to provide the shaft with a digital resolver which is able to precisely define its rotational position at any instant in time. Then, by storing the measured values along with the rotational position of the shaft when the value was measured, the signal can later be amplified when that specific location on the shaft is disposed proximate the cathode. In this way, the action of the cathode (i.e. the amplified voltage being supplied to the cathode) can be delayed a variable length of time until the resolver indicates that that particular position on the shaft is coincident with the metal removing means, such as the cathode, and the shaft's material can be removed at precisely the correct spot and at precisely the correct rate.
Many applications of the present invention do not require any additional electronic or computer means for coordinating the operation of the electrochemical machining device and the measuring means. For example, in the case where the electochemical machining device has its cathode positioned at exactly the same place as the measuring device obviously no coordinating system is required. Also, in shafts which have an even number of lobes or deviation high points, the cathode of the electrochemical machining apparatus can be placed an arcuate distance around the shaft which is diametrically opposite from the probe of the measuring device. It has been found that, in shafts with an even number of lobes, the machining of a lobe in response to a measurement of a diametrically opposite lobe is satisfactory for purposes of rounding the shaft. It has also been found that, with shafts having an odd number of lobes, the placement of the cathode at the point of contact between the bracket and the shaft is appropriate for removing high points. In this type of application, the measuring device appears to read a low point because of the fact that a high point is passing under one of the two contact points of the bracket. If the amplified signal from the measuring device is appropriately inverted, the high points or lobes of a three lobed shaft can be adequately rounded with this type of configuration.
The present invention provides an apparatus for removing material from the high points on the cylindrical surface of a shaft and, thus, improving its roundness. It incorporates a means for measuring the distance between a region on the cylindrical surface of the shaft and a reference line which, in the preferred embodiment, is a chordal line drawn between two points of contact which exist between a bracket which holds the measuring device and the workpiece shaft. A metal removing apparatus, such as an electrochemical machining cathode, is disposed proximate the shaft and a flow of electrolyte is provided through the gap which exists between the cathode and the shaft's surface. An electric signal from the measuring device is amplified and supplied to the cathode. By making the electrical signal analogous to the value measured by the measuring device, its amplification can produce an electrial voltage which can be supplied to the cathode and, therefore, the rate of metal removal at the location of the cathode can be directly related to the measurement of the radial deviation of the shaft's surface.